Just gonna say my 2¢ on this OC matter:
I'd consider my gal Maeria a character I made, but I wouldn't go so far as to say she's an OC. I made her in WoW but she has her own story and background and continues to build upon that when I play her. Though the story is fully built upon existing WoW Lore. I say you can call whatever you want an OC to be an OC, as long as it's not literally a blatant copy of something pre-existing (aka here's my character BLONIC). It's a line based on perception. If you put any decent amount of effort to make a character and put personality into it, call it whatever you want.
But in Flat's regard I don't have enough information to have a valued opinion towards his character. If it's literally just a drawing of his Spiral Knight's character, I wouldn't say it's an OC. It's fan art. But if he took the time to make a real CHARACTER out of it, that's different.